Tomato blight infests Pontiac

On our way home from Toronto on September 1, Eric and I stopped by Buster and
Anita Draper’s farm to buy some sweet corn. As usual, we fell to chatting, and
during the course of the conversation, Buster asked us whether we had lost our
tomatoes.

What? We were astonished, and told him that all the plants were fine when we
left. Our concern grew as he noted that their plants had been, too, until they
suddenly noticed a dramatic change.

Their tomatoes have been destroyed by blight. Tomatoes suddenly turn black,
so Buster said, or appear okay on the outside but, when cut open, must be thrown
away as they are black inside.

Despite the fact that night was already upon us, as soon as we arrived home
Eric went out into the vegetable garden armed with a flashlight. Would our
plants be all right?

Yes. All our tomato plants were fine.

Since then, we have monitored them but all are doing well and providing us
with superb, tasty tomatoes.

But other home gardeners in the area are experiencing exactly the same
devastating situation as the Drapers. One neighbour painstakingly grew every
plant from seed, only to have to pull out every single plant due to blight.
Others are finding that a few plants are okay, while others are infected.

Why are our plants okay but our close neighbours’ are not? We bought our
plants at Pinks, as did some of our friends, so what’s going on?

This BC Government’s Ministry of Agriculture and Foods site displays an
information sheet called “Late Blight Disease on Home Garden Tomatoes”
written by Janice Elmhirst, BCMAF Plant Pathologist. Here are some of her
comments.

First, what is blight?

“...caused by Phytophthora infestans. Phytophthora
infestans is not a bacterium or a virus. It belongs to a group of organisms
called “protists” officially, although they are still commonly referred to
as “fungi”. They are also called “water moulds” because they produce
spores and cause infection only when free water is present on the plants.”

Because the plants are related, late blight also affects potatoes and
eggplant, as well as the wild plant called “deadly nightshade.”

What parts of the tomato plant are affected?

Blight affects stems, leaves, petioles as well as fruit. Black or dark brown
lesions appear. On the fruit, she notes, “infection causes a brown/black,
leathery rot. It may become soft and mushy if invaded by secondary organisms.”

How does this late blight spread from plant to plant, garden to garden?

Ms. Elmhirst advises, “In cool, wet or humid weather the fungus produces
spores called “sporangia”. These spores can travel up to 20 kilometers in
wind-blown rain. Rain-spread spores can cause infection even in a garden where
tomatoes or potatoes have not been grown before.

“Sporangia can also move in ground water, runoff or in watering splash from
plant to plant in the garden. If they are contained in a water droplet which
does not dry up for a few hours, they will germinate to produce other tiny
spores called “zoospores” which swim through the water, attach themselves to
the leaf or stem tissue and cause infection.

“Some older literature may state that late blight is not a soil-borne
disease, that it needs living plant tissue to survive. It is true, that when
only a single mating type of P. infestans occurs on a plant, it does not
survive in bare soil, well-rotted compost or organic mulch. However, this
statement can be misleading to gardeners, since the organism can survive mild
winters on small bits of un-rotted or un-frozen plant debris in the soil.”

Okay, so the spores can travel on the wind for 20 kilometres. And the
organism can also get into the ground water.

How is it that our garden is free of this disease when we are not even a
kilometre away from our nearest neighbours whose plants are infected?

It is as I expected: our garden is located in a sunny spot. But more than
this, the answer seems to lie in our soil, which is sand, not clay or loam.
Finally, because our plants experienced verticillium wilt and other bacterial
diseases several years ago, we’ve been burning all our tomato vines and other
garden detritus, not putting it all into our compost. Finally, the other thing
we’ve done is grow plants from disease-resistant seeds.

However, what does the plant scientist advise about why adjacent neighbours
won’t experience the same blight conditions?

“Rain-spread spores can cause spotty outbreaks. Also, disease development
depends on the temperature and humidity around your plants. Plants in warmer,
drier, sunny spots will have less disease. For example, plants that receive the
morning sun will dry off more quickly from nightly dew and fog. Plants grown in
a high moisture-holding soil or planting mix will have a cooler and more humid
environment which is more favourable for disease, than plants grown on a sandy
soil or plastic mulch.”

Another point that my neighbours have been commenting on is that they have
tried to rescue tomatoes by picking them when they are green, and letting them
ripen indoors. Alas, the perfect-looking fruit rot in any case. Why, they
wonder, does this happen? Once again, Ms Elmhirst answers:

“In wet weather, green fruit may have been infected already, or be carrying
spores on the surface. As the fruit ripens, rot develops. Some gardeners report
that washing green fruit in soap and water after picking, or dipping green fruit
in a 10% bleach solution (1 part household bleach to 9 parts water) followed by
a soap and water wash, reduces fruit rot during ripening.”

Finally, Ms. Elmhirst advises rather depressingly that blight is getting
worse. “Every year, temperature and rainfall affect the timing and severity of
late blight disease. But, in recent years, new, more aggressive strains, in
combination with cool, wet summers, have led to an escalating blight problem.”

This all sounds extremely depressing but there are some simple garden
solutions we can all start to practise which will keep our plots healthier. Next
week, I’ll discuss these.